arm64: mm: Cast start/end markers to char *, not u64

There are a few memset() calls in map_kernel.c that cast marker-symbol
addresses to u64 in order to perform pointer subtraction (range size
computation).

Cast them with (char *) instead, aligning with idiomatic C pointer
arithmetic.

This patch provably has no effect at runtime: I have verified that
.text of vmlinux is identical after this change.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>